The conventional fabrication sequence for making microminiature electronic devices, such as semiconductor integrated circuits, utilizes lithographic techniques. Such techniques are especially useful for devices such as integrated circuit memory, integrated circuit logic, and magnetic bubble memory technology, where there is a need to fabricate devices having small features with low defect density. In the effort to reduce costs and to increase the efficiency of devices such as semiconductor integrated circuits as well as make new devices, processes have improved and become increasingly sophisticated in the past decade. These improvements have resulted in both more devices on a single semiconductor body (commonly termed a "chip") and smaller device sizes.
A typical lithographic process reproduces a pattern in radiation-responsive materials, commonly termed resists, which coat the surface of a semiconductor wafer, each wafer ultimately yielding many chips. The resist is exposed to radiation with a mask between the resist and the radiation source. The resolution ultimately obtained in the pattern-delineated resist is thus limited by, among other factors, the mask. In order for the semiconductor wafer to be used efficiently, so as to obtain a low cost per bit or function performed, processes have been developed which yield a very accurate mask for patterning the resist, and hence, obtaining a faithful reproduction of the mask pattern in the resist.
Although known processes, such as photo and electron beam lithography, have been and are being used commercially, X-ray lithography is considered promising since, first, it offers possibilities of obtaining fine features and high throughput and second, it is potentially less expensive than electron beam lithography. However, the resolution obtained in the pattern-delineated resist is, of course, limited by the integrity of the mask structure and tolerances of the mask features. Hence, an effort has been devoted to developing an X-ray lithographic process which includes the fabrication of a suitable X-ray mask structure. A typical mask structure comprises an X-ray transparent film substrate in conjunction with an X-ray absorbing overlay pattern.
For example, in U.S. Pat. No. 3,892,973, issued on Feb. 15, 1976 to G. A. Coquin, J. R. Maldonado, and D. Maydan, entitled "Mask Structure for X-ray Lithography" and assigned to Bell Telephone Laboratories, Incorporated, an X-ray lithographic process suitable for replication of patterns having fine features is described. In accordance with an exemplary process, a thin sheet of Mylar polyester film, stretched over and bonded to a support member, is used as the film substrate for a metallic layer in which an X-ray absorptive pattern is formed, resulting in the mask.
The deposition and patterning of such a metallic pattern, on a stretched film, however, may introduce undesirable stresses in the film which result in distortion of the X-ray absorptive pattern. An advantageous mask structure which minimizes such stresses is disclosed in U.S. Pat. No. 4,037,111 issued on June 8, 1976 to G. A. Coquin, J. R. Maldonado, D. Maydan, and S. R. Somekh and assigned to Bell Telephone Laboratories, Incorporated. The mask substrate comprises a thin sheet of polyimide film stretched over and bonded to a 2- or 3-dimensionally stable support member. Deposited on the film is a three-layer metallization structure which minimizes stress-induced distortions in the X-ray absorptive pattern. In some instances, however, it has been found that thermally-induced dimensional changes in the film cause distortions in the metallic pattern formed on the film.
In U.S. Pat. No. 4,171,489, issued to A. C. Adams, C. D. Capio, H. J. Levinstein, A. K. Sinha, and D. N. Wang on Sept. 13, 1978, entitled "Radiation Mask Structure" and assigned to Bell Telephone Laboratories, Incorporated, another mask structure for X-ray lithography is described. The mask comprises a substrate made of a film of boron nitride, bonded to a support frame, on which an X-ray-absorptive material is deposited. Such a mask exhibits an advantageously and substantially distortion-free characteristic which is obtained through either low or high pressure CVD (chemical vapor deposition) techniques. However, since the film comprises boron nitride, which is exceedingly fragile, the mask is sometimes difficult to handle and is prone to breakage.
In U.S. Pat. No. 4,253,029 issued to M. P. Lepselter, H. J. Levinstein, and D. Maydan on May 23, 1979, and assigned to Bell Telephone Laboratories, Incorporated, there is described a mask substrate comprising a composite structure of a boron nitride member under tension and coated with a polyimide layer; furthermore, an X-ray-absorptive material is formed thereon. This substrate is mechanically strong and desirably X-ray transparent, but still has some undesirable distortions due to stress.
In all the described masks, the supporting inorganic film (e.g., boron nitride film) should be under slight tension so as to avoid the wrinkling or distortions that result when it is under compression. It has been found experimentally that the desired stress be within the range from 0.1 to 0.5.times.10.sup.9 dynes/cm.sup.2. For stresses below this range, the X-ray mask structure exhibits a tendency to wrinkle and distort; and for stresses above this range, an X-ray mask structure exhibits a tendency to break. Hence, it is desirable to fabricate masks with stress within this range.
Chemical vapor deposition, which can be defined as a material synthesis method in which vapor phase constituents react to form a solid film on a surface, has been increasingly used in the formation of substrates for X-ray absorptive patterns used in fabrication of X-ray masks. The growth of thin films by chemical vapor deposition (CVD) has become one of the most important methods of thin film formation because its high versatility permits depositing a very large variety of compounds at relatively low temperatures, in the form of either vitreous or crystalline layers having a high degree of perfection and purity.
In their paper "The Chemical Deposition of Boron-Nitrogen Films," published in the Journal of Electrochemical Society in February 1980, A. C. Adams and C. D. Capio, of Bell Telephone Laboratories, Incorporated, described the deposition process of films of a boron-nitrogen compound with a composition of approximately B.sub.6 NH.sub.x, at reduced pressure, by reacting diborane and ammonia at temperatures between 250 degrees and 600 degrees C. It was found that x was a function of the deposition temperature. Through infrared spectra, they also observed that the films deposited in the low pressure chemical vapor deposition (LPCVD), reduced pressure reactor at 340 degrees C. had a lower stress than comparable films deposited at atmospheric pressure at 700 degrees C.
LPCVD of BN has been used in making masks for X-ray lithography. However, it has been found that deposition of large numbers of such films in one batch by LPCVD often leads to stresses within the deposited films that range from high compressive to high tensile, rendering a large percentage (as high as 90 percent) of the masks less suitable for commercial use as their stresses are outside the desired range.
Although the above methods are generally suitable for fabricating mask structures, it would be desirable to have a method for fabricating a mask structure for an X-ray lithographic system which could measure the as-deposited stress in the film and if it is outside a desired range, bring it into a desired range so that the structure would be more useful.